1 CH21 8/1/01 3:18 PM Page 1103 CHAPTER 21 Power Supply and Chassis/Case
2 1104 Chapter 21 Power Supply and Chassis/Case Considering the Importance of the Power Supply The power supply is not only one of the most important parts in a PC, but it is unfortunately also the most overlooked. In the words of a famous comedian, the power supply gets no respect! People spend hours discussing their processor speeds, memory capacity, disk storage capacity and speed, video adapter performance, monitor size, and so forth, but rarely even mention or consider their power supply. When a system is put together to meet the lowest possible price point, what component do you think the manufacturer will skimp on? Yes, the power supply. To most people, the power supply is a rather nondescript, unglamorous metal box that sits inside their systems, something to which they pay virtually no attention at all. The few who do pay any mind seem concerned only with how many watts of power it is rated to put out (even though no practical way exists to verify those ratings), without regard as to whether the power being produced is clean and stable, or whether it is full of noise, spikes, and surges. I have always placed great emphasis on selecting a power supply for my systems. I consider the power supply the core of the system and am willing to spend more to get a better unit. The power supply function is critical because it supplies electrical power to every other component in the system. In my experience, the power supply is also one of the most failure-prone components in any computer system, especially due to the fact that so many system assemblers use the cheapest ones they can find. A malfunctioning power supply can not only cause other components in the system to malfunction, but it also can damage the other components in your computer by delivering an improper or erratic voltage. Because of its importance to proper and reliable system operation, you should understand both the function and limitations of a power supply, as well as its potential problems and their solutions. This chapter covers the power supply in detail. I focus on the electrical functions of the supply and the mechanical form factors and physical designs that have been used in PC systems in the past, as well as today. Because the physical shape (form factor) of the power supply relates to the case, some of this information also relates to the type of chassis or case you have. Primary Function and Operation The basic function of the power supply is to convert the type of electrical power available at the wall socket to the type the computer circuitry can use. The power supply in a conventional desktop system is designed to convert either 115-volt (nominal) 60Hz AC (alternating current) or 230V (nominal) 50Hz AC power into +3.3V, +5V, and +12V DC (direct current) power. Some power supplies require you to switch between the two input ranges, whereas others auto-switch. Positive DC Voltages Usually, the digital electronic components and circuits in the system (motherboard, adapter cards, and disk drive logic boards) use the +3.3V or +5V power, and the motors (disk drive motors and any fans) use the +12V power. Table 21.1 lists these devices and their power consumptions. Table 21.1 Voltage Power Consumption Ratings for PC Devices Devices Powered +3.3V Chipsets, DIMMs, PCI/AGP cards, miscellaneous chips +5V Disk drive logic, SIMMs, PCI/AGP cards, ISA cards, voltage regulators, miscellaneous chips +12V Motors, voltage regulators (high output)
3 Primary Function and Operation Chapter The power supply must deliver a good, steady supply of DC power so that the system can operate properly. Devices that run on voltages other than these must be powered by onboard voltage regulators. For example, RIMMs run on 2.5V that is supplied by an onboard regulator, and processors are supplied by a voltage regulator module (VRM) that normally is built into the motherboard as well. Note When Intel began releasing processors that required a +3.3V power source, power supplies that supplied the additional output voltage were not yet available. As a result, motherboard manufacturers began adding voltage regulators to their boards, which converted +5V current to +3.3V for the processor. When other chips began using 3.3V as well, Intel created the ATX power supply specification, which supplied 3.3V to the motherboard. DIMMs (Dual In-line Memory Modules) also run on +3.3V as supplied by the power supply. You would think that having 3.3V direct from the power supply would have eliminated the need for onboard voltage regulators, but by that time, processors began running on a wide variety of voltages lower than 3.3V. Motherboard manufacturers then included adaptable regulator circuits called Voltage Regulator Modules (VRMs) to accommodate the widely varying processor voltage requirements. See CPU Operating Voltages, p. 97. Negative DC Voltages If you look at a specification sheet for a typical PC power supply, you can see that the supply generates not only +3.3V, +5V, and +12V, but also 5V and 12V. The positive voltages seemingly power everything in the system (logic and motors), so what are the negative voltages used for? The answer is, not much! Some of the power supply designs, such as the SFX (Small form factor) design, no longer include the 5V output for that reason. The only reason it has remained in most power supply designs is that 5V is required on the Industry Standard Architecture (ISA) bus for full backwardcompatibility. Although 5V and 12V are supplied to the motherboard via the power supply connectors, the motherboard normally uses only the +3.3V, +5V, and +12V. The 5V is simply routed to the ISA bus on pin B5 so any ISA cards can use it. Today, though, not many do. However, as an example, the analog data separator circuits found in older floppy controllers do use 5V. The motherboard logic normally doesn t use 12V either; however, it might be used in some board designs for serial port or LAN circuits. Note The load placed on the 12V output by an integrated LAN adapter is very small. For example, the integrated 10/100 Ethernet adapter in the Intel D815EEAL motherboard uses only 10mA of +12V and 10mA of 12V (0.01 amps each) to operate. Although older serial port circuits used +/ 12V outputs, today most run on only +3.3V or +5V. The main function of the +12V power is to run disk drive motors as well as the higher-output processor voltage regulators in some of the newer boards. Usually, a large amount of +12V current is available from the power supply, especially in those designed for systems with a large number of drive bays (such as in a tower configuration). Besides disk drive motors and newer CPU voltage regulators, the +12V supply is used by any cooling fans in the system which, of course, should always be running. A single cooling fan can draw between 100mA and 250mA ( amps); however, most newer fans use the lower 100mA figure. Note that although most fans in desktop systems run on +12V, portable systems can use fans that run on +5V, or even +3.3V.
4 1106 Chapter 21 Power Supply and Chassis/Case Most systems with newer motherboard form factors, such as the ATX, micro-atx, or NLX, include another special signal. This feature, called PS_ON, can be used to turn the power supply (and thus the system) on or off via software. It is sometimes known as the soft-power feature. PS_ON is most evident when you use it with an operating system, such as Windows 9x, that supports the Advanced Power Management (APM) or Advanced Configuration and Power Interface (ACPI) specification. When you select the Shut Down the Computer option from the Start menu, Windows automatically turns off the computer after it completes the OS shutdown sequence. A system without this feature only displays a message that it s safe to shut down the computer. The Power_Good Signal In addition to supplying electrical power to run the system, the power supply also ensures that the system does not run unless the power supplied is sufficient to operate the system properly. In other words, the power supply actually prevents the computer from starting up or operating until all the power supply voltages are within the proper ranges. The power supply completes internal checks and tests before allowing the system to start. If the tests are successful, the power supply sends a special signal to the motherboard, called Power_Good. This signal must be continuously present for the system to run. Therefore, when the AC voltage dips and the power supply cannot maintain outputs within regulation tolerance, the Power_Good signal is withdrawn (goes low) and forces the system to reset. The system will not restart until the Power_Good signal returns. The Power_Good signal (sometimes called Power_OK or PWR_OK) is a +5V (nominal) active high signal (with variation from +2.4V through +6.0V generally being considered acceptable) that is supplied to the motherboard when the power supply has passed its internal self tests and the output voltages have stabilized. This normally takes place anywhere from 100ms to 500ms ( seconds) after you turn on the power supply switch. The power supply then sends the Power_Good signal to the motherboard, where the processor timer chip that controls the reset line to the processor receives it. In the absence of Power_Good, the timer chip holds the reset line on the processor, which prevents the system from running under bad or unstable power conditions. When the timer chip receives the Power_Good signal, it releases the reset, and the processor begins executing whatever code is at address FFFF:0000 (usually the ROM BIOS). If the power supply cannot maintain proper outputs (such as when a brownout occurs), the Power_Good signal is withdrawn, and the processor is automatically reset. When the power output returns to its proper levels, the power supply regenerates the Power_Good signal and the system again begins operation (as if you had just powered on). By withdrawing Power_Good before the output voltages fall out of regulation, the system never sees the bad power because it is stopped quickly (reset) rather than being allowed to operate using unstable or improper power levels, which can cause memory parity errors and other problems. Note You can use the Power_Good feature as a method of implementing a reset switch for the PC. The Power_Good line is wired to the clock generator circuit, which controls the clock and reset lines to the microprocessor. When you ground the Power_Good line with a switch, the timer chip and related circuitry reset the processor. The result is a full hardware reset of the system. Upgrading and Repairing PCs, 6th Edition, which is located on this book s CD, contains instructions for making and installing a reset switch. See Parity and ECC, p On pre-atx systems, the Power_Good connection is made via connector P8-1 (P8 ) from the power supply to the motherboard. ATX and later systems use pin 8 of the 20-pin connector, which is normally a gray wire.
5 Power Supply Form Factors Chapter A well-designed power supply delays the arrival of the Power_Good signal until all the voltages stabilize after you turn on the system. Badly designed power supplies, which are found in many low-cost systems, often do not delay the Power_Good signal properly and enable the processor to start too soon. (The normal Power_Good delay is seconds.) Improper Power_Good timing also causes CMOS memory corruption in some systems. Note If you find that a system consistently fails to boot up properly the first time you turn on the switch, but that it subsequently boots up if you press the reset or Ctrl+Alt+Delete warm boot command, you likely have a problem with the Power_Good timing. You should install a new, higher-quality power supply and see whether that solves the problem. Some cheaper power supplies do not have proper Power_Good circuitry and might just tie any +5V line to that signal. Some motherboards are more sensitive to an improperly designed or improperly functioning Power_Good signal than others. Intermittent startup problems are often the result of improper Power_Good signal timing. A common example is when you replace a motherboard in a system and then find that the system intermittently fails to start properly when you turn on the power. This can be very difficult to diagnose, especially for the inexperienced technician, because the problem appears to be caused by the new motherboard. Although it seems as though the new motherboard is defective, it usually turns out that the power supply is poorly designed. It either cannot produce stable enough power to properly operate the new board or has an improperly wired or timed Power_Good signal (which is more likely). In these situations, replacing the supply with a higherquality unit, in addition to the new motherboard, is the proper solution. Power Supply Form Factors The shape and general physical layout of a component is called the form factor. Items that share a form factor are generally interchangeable, at least as far as their sizes and fits are concerned. When designing a PC, the engineers can choose to use one of the popular standard PSU (power supply unit) form factors, or they can elect to build their own. Choosing the former means that a virtually inexhaustible supply of inexpensive replacement parts will be available in a variety of quality and power output levels. Going the custom route means additional time and expense for development. In addition, the power supply is unique to the system and available only from the original manufacturer. If you can t tell already, I am a fan of the industry-standard form factors! Having standards and then following them allows us to upgrade and repair our systems by easily replacing physically (and electrically) interchangeable components. Having interchangeable parts means that we have a better range of choices for replacement items, and the competition makes for better pricing, too. In the PC market, IBM originally defined the standards, and everybody else copied them. This included power supplies. All the popular PC power supply form factors up through 1995 were based on one of three IBM models, including the PC/XT, AT, and PS/2 Model 30. The interesting thing is that these three power supply definitions all had the same motherboard connectors and pinouts; where they differed was mainly in shape, maximum power output, the number of peripheral power connectors, and switch mounting. PC systems using knock-offs of one of those three designs were popular through 1996 and beyond, and some systems still use them today. Intel gave the power supply a new definition in 1995 with the introduction of the ATX form factor. ATX became popular in 1996 and started a shift away from the previous IBM-based standards. ATX and the related standards that followed have different connectors with additional voltages and signals that allow systems with greater power consumption and additional features that would otherwise not be possible with the AT style supplies.
6 1108 Chapter 21 Power Supply and Chassis/Case Technically, the power supply in your PC is described as a constant voltage half-bridge forward converting switching power supply: Constant voltage means that the power supply puts out the same voltage to the computer s internal components, no matter what the voltage of AC current running it or the capacity (wattage) of the power supply. Half-bridge forward converting switching refers to the design and power regulation technique used by most suppliers. This design is commonly referred to as a switching supply. Compared to other types of power supplies, this design provides an efficient and inexpensive power source and generates a minimum amount of heat. It also maintains a small size and a low price. Note Although two power supplies can share the same basic design and form factor, they can differ greatly in quality and efficiency. Later in this chapter, you ll learn about some of the features and specifications to look for when evaluating PC power supplies. Seven main power supply physical form factors have existed that can be called industry standard. Five of these are based on IBM designs, whereas two are based on Intel designs. Of these, only three are used in most modern systems; the others are pretty much obsolete. Note that although the names of the power supply form factors seem to be the same as those of motherboard form factors, the power supply form factor is more related to the system chassis (case) than the motherboard. That is because all the form factors use one of only two types of connector designs, either AT or ATX. For example, all PC/XT, AT, and LPX form factor supplies use the same pair of six-pin connectors to plug into the motherboard and will therefore power any board having the same type of power connections. Plugging into the motherboard is one thing, but for the power supply to physically work in the system, it must fit the case. The bottom line is that it is up to you to make sure the power supply you purchase not only plugs into your motherboard but also fits into the chassis or case you plan to use. Table 21.2 shows these power supply form factors, their connection types, and associated motherboards. Table 21.2 Power Supply Connector Types and Form Factors Obsolete PS Connector Associated MB Form Form Factors Originated From Type Factors PC/XT style IBM PC, PC-XT (1981/1983) AT PC/XT, Baby-AT AT/Desk style IBM PC-AT (1984) AT Fullsize AT, Baby-AT AT/Tower style IBM PC-AT (1984) AT Fullsize AT, Baby-AT Baby-AT style IBM PC-AT (1984) AT Fullsize AT, Baby-AT Modern PS Connector Associated MB Form Form Factors Originated From Type Factors LPX style* IBM PS/2 Model 30 (1987) AT Baby-AT, Mini-AT, LPX ATX style Intel ATX, ATX12V (1985/2000) ATX ATX, NLX, Micro-ATX SFX style Intel SFX (1997) ATX Flex-ATX, Micro-ATX *Note: LPX is also sometimes called Slimline or PS/2.
7 Power Supply Form Factors Chapter Each of these power supply form factors is available in numerous configurations and power output levels. The LPX form factor supply had been the standard used on most systems from the late 1980s to mid-1996, when the ATX form factor started to gain in popularity. Since then, ATX has become by far the dominant form factor for power supplies, with the new SFX style being added as an ATX derivative for use in very compact systems that mainly use Flex-ATX sized boards. The earlier IBM-derived form factors have been largely obsolete for some time now. See Motherboard Form Factors, p IBM s PC and XT systems (circa 1981 and 1983, respectively) used the same power supply form factor; the only difference was that the XT supply had more than double the power output capability. Because they were identical in external appearance and the type of connectors they used, you easily could install the higher output XT supply as an upgrade for a PC system; thus, the idea of upgrading the power supply was born. The tremendous popularity of the original PC and XT systems led a number of manufacturers to begin building systems that mimicked their shapes and layouts. These clones or compatibles as they have been called, could interchange virtually all components with the IBM systems, including the power supply. Numerous manufacturers then began producing components that followed the form factor of these systems. The PC/XT power supply and connectors are shown in Figure This form factor, however, is not used in modern systems today. 48mm Pin 4 +12V(Yellow) 120mm ON Pin 4 +12V(Yellow) OFF Pin 4 +12V(Yellow) 12mm mm 190mm Pin 4 +12V(Yellow) 120mm 100mm P8 5 6 Pin 6 5V (White) 8mm 12 20mm 210mm 222mm P9 5 6 Pin 6 PWR_OK (Orange) +12V (Yellow) 12V (Blue) Figure 21.1 PC/XT form factor power supply. AT/Desk Style The AT desktop system introduced by IBM in August 1984, had a larger power supply and a form factor different from the original PC/XT. This system was rapidly cloned by other manufacturers and represented the basis for many subsequent IBM-compatible designs. The power supply used in these systems was called the AT/Desktop-style power supply (see Figure 21.2). Hundreds of manufacturers began making motherboards, power supplies, cases, and other components that were physically interchangeable with the original IBM AT. This form factor is no longer used today.
8 1110 Chapter 21 Power Supply and Chassis/Case Pin 4 +12V(Yellow) 150mm ON OFF Pin 4 +12V(Yellow) Pin 4 +12V(Yellow) 150mm mm Pin 4 +12V(Yellow) 13mm Pin 4 +12V(Yellow) 150mm 131mm 6mm 28mm 35mm + + 7mm 6 144mm 16mm 47mm 213mm 8.35"x5.9"x5.9" P P8 5 6 Pin 6 Pin 6 PWR_OK (Orange) +12V (Yellow) 12V (Blue) 5V (White) Figure 21.2 AT/Desktop form factor power supply. AT/Tower Style The AT/Tower configuration was basically a full-sized, AT-style desktop system running on its side. The tower configuration was not new; in fact, even IBM s original AT had a specially mounted logo that could be rotated when you ran the system on its side in the tower configuration. The type of power supply used in most of the AT tower systems was identical to that used in a desktop system, except for the location of the power switch. On the original AT/Desktop systems, the power switch was built into the side of the power supply (usually in the form of a large toggle switch). AT/Tower systems instead used an external switch attached to the power supply through a four-wire cable. A full-sized AT power supply with a remote switch is called an AT/Tower form factor power supply and is identical to the AT/Desktop supply in size and dimensions. The only difference is the use of an external switch (see Figure 21.3). This form factor is still used today in large server chassis that run AT form factor motherboards. Baby-AT Style Another type of AT-based form factor is the so-called Baby-AT, which is a shortened version of the full-sized AT form factor. The power supply in these systems is shortened in one dimension but matches the AT design in all other respects. Baby-AT style power supplies could fit in place of the larger AT/Desktop style supply; however, the full-sized AT/Tower supply would not fit in the Baby-AT chassis (see Figure 21.4). Because the Baby-AT PSU performed the same functions as the AT-style power supply but was in a smaller package, it became a common form factor until it was overtaken by later designs. This form factor is rarely used today. LPX Style The next power supply form factor to gain popularity was the LPX style, also called the PS/2 type, Slimline, or slim style (see Figure 21.5). The LPX-style power supply has the exact same motherboard and disk drive connectors as the previous standard power supply form factors; it differs mainly in the shape. LPX systems were designed to have a smaller footprint and a lower height than AT-sized systems. These computers used a different motherboard configuration that mounts the expansion bus
9 Power Supply Form Factors Chapter slots on a riser card that plugs into the motherboard. The expansion cards plug into this riser and are mounted sideways in the system, parallel to the motherboard. Because of its smaller case, an LPX system needed a smaller power supply. The power supply designed for LPX systems is smaller than the Baby-AT style in every dimension and takes up less than half the space of its predecessor Pin 4 +12V(Yellow) 150mm Pin 4 +12V(Yellow) Pin 4 +12V(Yellow) 150mm mm Pin 4 +12V(Yellow) 13mm Pin 4 +12V(Yellow) 150mm 131mm 6mm mm 16mm 213mm 8.35"x5.9"x5.9" 28mm 35mm 7mm 47mm P P8 5 6 Pin 6 Pin 6 PWR_OK (Orange) +12V (Yellow) 12V (Blue) 5V (White) Figure 21.3 AT/Tower form factor power supply V (Yellow) 150mm ON OFF Pin 4 +12V(Yellow) Pin 4 +12V(Yellow) 150mm mm Pin 4 +12V(Yellow) 13mm Pin 4 +12V(Yellow) 150mm 131mm 6mm P9 5 6 Pin 6 PWR_OK (Orange) +12V (Yellow) 12V (Blue) 6.5"x5.9"x5.9" 8 144mm 165mm P8 5 6 Pin 6 5V (White) Figure 21.4 Baby-AT form factor power supply. Note IBM used this type of power supply in some of its PS/2 systems in the late 1980s; hence it is sometimes called a PS/2-type supply.
10 1112 Chapter 21 Power Supply and Chassis/Case V (Yellow) Pin 4 +12V (Yellow) 86mm Pin 4 +12V (Yellow) 140mm mm Pin 4 +12V (Yellow) 6mm 64mm 86mm P9 5 6 Pin 6 PWR_OK (Orange) +12V (Yellow) 12V (Blue) 16mm mm 150mm + 30mm P8 5 6 Pin 6 5V (White) 5.9"x5.5"x3.4" Figure 21.5 LPX form factor power supply. As with the Baby-AT design in its time, the LPX power supply does the same job as its predecessor but comes in a smaller package. The LPX power supply quickly found its way into many manufacturers systems, soon becoming a de facto standard. This style of power supply became the staple of the industry for many years, coming in everything from low-profile systems using actual LPX motherboards to full-size towers using Baby-AT or even full-size AT motherboards. It still is used in some PCs produced today; however, since 1996 the popularity of LPX has been overshadowed by the increasing popularity of the ATX design. ATX Style One of the newer standards in the industry today is the ATX form factor (see Figure 21.6). The ATX specification, now in version 2.03, defines a new motherboard shape, as well as a new case and power supply form factor. The shape of the ATX power supply is based on the LPX design, but some important differences are worth noting. One difference is that the ATX specification originally called for the fan to be mounted along the inner side of the supply, where it could draw air in from the rear of the chassis and blow it inside across the motherboard. This kind of airflow runs in the opposite direction as most standard supplies, which exhaust air out the back of the supply through a hole in the case where the fan protrudes. The idea was that the reverse flow design could cool the system more efficiently with only a single fan, eliminating the need for a fan (active) heatsink on the CPU. Another benefit of the reverse-flow cooling is that the system would run cleaner, more free from dust and dirt. The case would be pressurized, so air would be continuously forced out of the cracks in the case the opposite of what happens with a negative pressure design. For this reason, the reverse-flow cooling design is often referred to as a positive-pressure-ventilation design. On an ATX system with reverse-flow cooling, the air would be blown out away from the drive because the only air intake would be the single fan vent on the power supply at the rear. For systems that operate in extremely harsh environments, you can add a filter to the fan intake vent to further ensure that all the air entering the system is clean and free of dust.
11 Power Supply Form Factors Chapter Pin 4 +12V (Yellow) +12V (Yellow) Pin 4 +12V (Yellow) 86mm +12V (Yellow) Pin 4 140mm mm Pin 4 +12V (Yellow) 1 6mm + + (Orange) (Orange) +3.3V +3.3V +3.3V (Orange) 12V (Blue) 86mm 64mm (Black) GND (Red) +5V GND PS_ON# (Black) (Green) 16mm (Black) GND (Red) +5V GND GND (Black) (Black) 5 115mm 150mm 30mm (Black) (Gray) GND PWR_OK 5V (White) (Purple) +5VSB (Yellow) +12V "x5.5"x3.4" ATX power connector pin out Figure 21.6 ATX form factor power supply, used with both ATX and NLX systems. Although this sounds like a good way to ventilate a system, the positive-pressure design needs to use a more powerful fan to pull the required amount of air through a filter and to pressurize the case. Also, if a filter is used, it must be serviced on a periodic basis depending on operating conditions, it can need changing or cleaning as often as every week. In addition, the heat load from the power supply on a fully loaded system heats up the air being ingested, blowing warm air over the CPU, reducing overall cooling capability. As newer CPUs create more and more heat, the cooling capability of the system becomes more critical. In common practice, it was found that using a standard negativepressure system with an exhaust fan on the power supply and an additional high-quality cooling fan blowing cool air right on the CPU is the best solution. For this reason, the ATX power supply specification has been amended to allow for either positive- or negative-pressure ventilation. Because a standard negative-pressure system offers the most cooling capacity for a given fan airspeed and flow, most of the newer ATX-style power supplies use the negative-pressure cooling system. The ATX specification was first released by Intel in In 1996, it became increasingly popular in Pentium and Pentium Pro based PCs, capturing 18% of the motherboard market. Since 1996, ATX has become the dominant motherboard form factor, displacing the previously popular Baby-AT. ATX and its derivatives are likely to remain the most popular form factor for several years to come. The ATX form factor addressed several problems with the power supplies used with Baby-AT and mini-at form factors. One is that the power supplies used with Baby-AT boards have two connectors that plug into the motherboard. If you insert these connectors backward or out of their normal sequence, you will fry the motherboard! Most responsible system manufacturers key the motherboard and power supply connectors so that you cannot install them backward or out of sequence. However, some vendors of cheaper systems do not feature this keying on the boards or supplies they use. The ATX form factor includes different power plugs for the motherboard to prevent users from plugging in their power supplies incorrectly. The ATX design features up to three motherboard power connectors that are definitively keyed, making plugging them in backward virtually impossible. The new ATX connectors also supply +3.3V, reducing the need for voltage regulators on the motherboard to power the chipset, DIMMs, and other +3.3V circuits.
12 1114 Chapter 21 Power Supply and Chassis/Case Besides the new +3.3V outputs, another set of outputs is furnished by an ATX power supply that is not normally seen on standard power supplies. The set consists of the Power_On (PS_ON) and 5V_Standby (5VSB) outputs mentioned earlier, known collectively as Soft Power. This enables features to be implemented, such as Wake on Ring or Wake on LAN, in which a signal from a modem or network adapter can actually cause a PC to wake up and power on. Many such systems also have the option of setting a wake-up time, at which the PC can automatically turn itself on to perform scheduled tasks. These signals also can enable the optional use of the keyboard to power the system on exactly like Apple systems. Users can enable these features because the 5V Standby power is always active, giving the motherboard a limited source of power even when off. Check your BIOS Setup for control over these features. NLX Style The NLX specification, also developed by Intel, defines a low-profile case and motherboard design with many of the same attributes as the ATX. In fact, for interchangeability, NLX systems were designed to use ATX power supplies, even though the case and motherboard dimensions are different. As in previous LPX systems, the NLX motherboard uses a riser board for the expansion bus slots. Where NLX differs is that it is a true (and not proprietary) standard. See Chapter 4, Motherboards and Buses, for more information on the NLX form factor. For the purposes of this discussion, NLX systems use ATX power supplies. The only real difference is that the supply plugs into the riser card and not the motherboard, enabling NLX motherboards to be more quickly and easily removed from their chassis for service. SFX Style Intel released the smaller Micro-ATX motherboard form factor in December of 1997, and at the same time also released a new smaller SFX (Small form factor) power supply design to go with it (see Figure 21.7). Even so, most Micro-ATX chassis used the standard ATX power supply instead. Then in March of 1999, Intel released the Flex-ATX addendum to the Micro-ATX specification, which was a very small board designed for low-end PCs or PC-based appliances. At this point, the SFX supply has found use in many new compact system designs. Figure 21.7 SFX style power supply (with 90mm top-mounted cooling fan). The SFX power supply is specifically designed for use in small systems containing a limited amount of hardware and limited upgradability. Most SFX supplies can provide 90 watts of continuous power (135 watts at its peak) in four voltages (+5, +12, 12, and +3.3V). This amount of power has proved to be sufficient for a small system with a processor, an AGP interface, up to four expansion slots, and three peripheral devices such as hard drives and CD-ROMs.
13 Power Supply Form Factors Chapter Although Intel designed the SFX power supply specification with the Micro-ATX and Flex-ATX motherboard form factors in mind, SFX is a wholly separate standard that is compliant with other motherboards as well. SFX power supplies use the same 20-pin connector defined in the ATX standard and include both the Power_On and 5V_Standby outputs. Whether you will use an ATX or SFX power supply in a given system is dependent more on the case or chassis than the motherboard. Each has the same basic electrical connectors; the main difference is which type of power supply the case is physically designed to accept. One limiting factor on the SFX design is that it lacks the 5V and so shouldn t be used with motherboards that have ISA slots (most Micro-ATX and Flex-ATX boards do NOT have ISA slots). SFX power supplies also won t have the Auxiliary (3.3V and 5V) or ATX12V power connectors, and therefore shouldn t be used with full-size ATX boards that require those connections. On a standard model SFX power supply, a 60mm diameter cooling fan is located on the surface of the housing, facing the inside of the computer s case. The fan draws the air into the power supply housing from the system cavity and expels it through a port at the rear of the system. Internalizing the fan in this way reduces system noise and results in a standard negative-pressure design. In many cases, additional fans might be needed in the system to cool the processor (see Figure 21.8). 60mm fan 9.0 X 3.2 cutout clearance under cutout minimum 4.5mm from inside cover OP wire harness location is at manufacturer s discretion X 5.0 cutout 12x1 clearance under cutout minimum 6.0mm from inside cover Venting holes OPTIONAL to outside of chassis No UNC-2B ten-sided hole Figure 21.8 SFX form factor power supply dimensions with a standard internal 60mm fan.
14 1116 Chapter 21 Power Supply and Chassis/Case For systems that require more cooling capability, a version that allows for a larger 90mm top-mounted cooling fan also is available. The larger fan provides more cooling capability and airflow for systems that need it (see Figure 21.9) OP wire harness location is at manufacturer s discretion mm Fan Airflow 11.0 X 5.0 cutout clearance under cutout minimum at 6.0mm from inside cover Venting holes OPTIONAL to outside of chassis X 3.2 cutout clearance under Airflow cutout minimum at 6.0mm from inside cover 31.8 No UNC-2B ten-sided hole Figure 21.9 SFX form factor power supply dimensions with an internal 90mm top-mounted fan. Motherboard Power Connectors Every PC power supply has special connectors that attach to the motherboard, giving power to the system processor, memory, and all slotted add-on boards (ISA, PCI, AGP). Attaching these connectors improperly can have a devastating effect on your PC, including burning up both your power supply and motherboard. The following sections detail the motherboard power connectors used by various power supplies.
15 Motherboard Power Connectors Chapter AT Power Supply Connectors Industry standard PC, XT, AT, Baby-AT, and LPX motherboards all use the same type of main power supply connectors. These supplies feature two main power connectors (P8 and P9), each with 6 pins that attach the power supply to the motherboard. Those two connectors are shown in Figure P8 5 6 Pin 6 PWR_OK (Orange) +12V (Yellow) 12V (Blue) P9 5 6 Pin 6 5V (White) Figure LPX/AT Main Power Connectors AT/LPX main P8/P9 (sometimes also called P1/P2) power connectors. All standard PC power supplies that use the P8 and P9 connectors have them installed end to end so that the two black wires (ground connections) on both power cables are next to each other. Note the designations P8 and P9 are not fully standardized, although most use those designations because that is what IBM used on the originals. Some power supplies have them labeled as P1/P2 instead. Because these connectors usually have a clasp that prevents them from being inserted backward on the pins on the motherboards, the major concern is getting the two connectors in the correct orientation side by side and also not missing a pin offset on either side. Following the black-to-black rule keeps you safe. You must take care, however, to make sure that no remaining unconnected motherboard pins exist between or on either side of the two connectors after you install them. A properly installed connector connects to and covers every motherboard power pin. If any power pins are showing on either side of the connectors, the entire connector assembly is installed incorrectly, which can result in catastrophic failure for the motherboard and everything plugged into it at the time of power-up. Figure shows the P8 and P9 connectors (sometimes also called P1/P2) in their proper orientation when connecting. Table 21.3 shows typical AT and LPX power supply connections.
16 1118 Chapter 21 Power Supply and Chassis/Case Always keep black connectors side-by-side when plugging into the motherboard P8 connector P9 connector Figure The P8/P9 power connectors (sometimes also called P1/P2) that connect an AT/LPX power supply to the motherboard. Table 21.3 AT/LPX Power Supply Connectors Connector Pin Signal Color Connector Pin Signal Color P8 (or P1) 1 Power_Good Orange P9 (or P2) 1 Ground Black (+5V) 2 +5V* Red 2 Ground Black 3 +12V Yellow 3 5V White 4 12V Blue 4 +5V Red 5 Ground Black 5 +5V Red 6 Ground Black 6 +5V Red *Older PC/XT motherboards and power supplies did not use this output (P8 pin 2). Color codes can vary from manufacturer to manufacturer;, the ones shown here are typical. Tip Although older PC/XT power supplies do not have any connection at connector P8 pin 2, you still can use them on ATtype motherboards, or vice versa. The presence or absence of the +5V on that pin has little or no effect on system operation because the remaining +5V wires usually can carry the load. Note that all the power supplies from the AT/Desk through the Baby-AT and LPX power supplies use the same pin configuration. ATX Main Power Connector The industry standard ATX power-supply to motherboard main connector is the Molex (or equivalent) 20-pin ATX style connector (see Figure 21.12). First used in the ATX form factor power
17 Motherboard Power Connectors Chapter supply, it also is used in the SFX form factor or any other ATX-based variations. This is a 20-pin keyed connector with pins configured as shown in Table The colors for the wires listed are those recommended by the ATX standard; however, they are not required for compliance to the specification, so they could vary from manufacturer to manufacturer. Note that I like to show these connector pinouts in a wire side view, which shows how the pins are arranged looking at the back of the connector (from the wire and not terminal side). This is because it shows how they would be oriented if you were back-probing the connector with the connector plugged in Motherboard Power connector on motherboard 1 Key Figure ATX style 20-pin motherboard main power connector, perspective view. Figure shows a view of the connector as if you were looking at it facing the terminal side. 1 (Orange) +3.3V (Orange) +3.3V (Black) GND (Red) +5V (Black) GND (Red) +5V (Black) GND (Gray) PWR_OK (Purple) +5VSB (Yellow) +12V +3.3V (Orange) 12V (Blue) PS_ON# (Green) 5V (White) 0 0 ATX/NLX Main Power Connector Figure ATX/NLX 20-pin main power connector, terminal side view.
18 1120 Chapter 21 Power Supply and Chassis/Case Table 21.4 ATX Main Power Supply Connector Pinout (Wire Side View) Color Signal Pin Pin Signal Color Orange* +3.3V V Orange Blue 12V V Orange Black GND 13 3 GND Black Green PS_On V Red Black GND 15 5 GND Black Black GND V Red Black GND 17 7 GND Black White 5V 18 8 Power_Good Gray Red +5V VSB (Standby) Purple Red +5V V Yellow *Might have a second orange or brown wire, used for +3.3V sense feedback used by the power supply to monitor 3.3V regulation. Note The ATX supply features several voltages and signals not seen before, such as the +3.3V, PS_On, and +5V_Standby. Therefore, adapting a standard LPX form factor supply to make it work properly in an ATX system, is difficult if not impossible even though the shapes of the power supplies themselves are virtually identical. However, because ATX is a superset of the older AT standard, you can use an adapter to allow an ATX power supply to connect to an older Baby-AT style motherboard. PC Power and Cooling (see the vendor list) sells this type of adapter. ATX Auxiliary Power Connector As motherboards and processors evolved, the need for power became greater. In particular, chipsets and DIMMs were designed to run on 3.3V, increasing the current demand at that voltage. In addition, most boards included CPU voltage regulators designed to convert +5V power into the unique voltage levels required by the processors the board supported. Eventually, the high current demands on the +3.3V and +5V outputs were proving too much for the number and gauge of the wires used. Melted connectors were becoming more and more common as these wires overheated under these loads. Finally, Intel modified the ATX specification to add a second power connector for ATX motherboards and supplies. The criteria was that if the motherboard needed more than 18A of +3.3V power, or more than 24A of +5V power, an auxiliary connector would be defined to carry the additional load. These higher levels of power are normally necessary in systems using 250-watt to 300-watt or greater supplies. The auxiliary connector (shown in Figure 21.14) is a 6-pin Molex-type connector, similar to one of the motherboard power connectors used on AT/LPX supplies. It is keyed to prevent misconnection. The pinouts of the auxiliary connector are shown in Table 21.5.
19 Motherboard Power Connectors Chapter Pin V (Orange) +3.3V (Orange) ATX Auxiliary Power Connector Figure ATX auxiliary power connector. Table 21.5 ATX Auxiliary Power Connector Pinout Pin Signal Color Pin Signal Color 1 Gnd Black V Orange 2 Gnd Black V Orange 3 Gnd Black 6 +5V Red If your motherboard does not feature a mating auxiliary connector, it probably wasn t designed to consume a large amount of power, and the auxiliary connector from the power supply can be left unconnected. If your power supply is rated at 250 watts or larger, you should ensure that it has this connector and that your motherboard is capable of accepting it. This eases the load on the main power connector. ATX12V Connector Power for the processor comes from a device called the voltage regulator module (VRM), which is built into most modern motherboards. This device senses the CPU voltage requirements (usually via sense pins on the processor) and calibrates itself to provide the proper voltage to run the CPU. The design of a VRM enables it to run on either 5V or 12V for input power. Most have used 5V over the years, but many are now converting to 12V because of the lower current requirements at that voltage. In addition, the 5V already might be loaded by other devices, whereas, typically, only drive motors use the 12V. Whether the VRM on your board uses 5V or 12V depends on the particular motherboard or regulator design. Many modern voltage regulator ICs are designed to run on anything from a 4V to a 36V input, so it is up to the motherboard designer as to how they will be configured. For example, I studied a system using an FIC (First International Computer) SD-11 motherboard, which used a Semtech SC1144ABCSW voltage regulator. This board design uses the +5V to convert to the lower voltage needed by the CPU. Most motherboards use voltage regulator circuits controlled by chips from Semtech ( or Linear Technology ( You can visit their sites for more data on these chips. That motherboard accepts an Athlon 1GHz Cartridge version (Model 2), which according to AMD has a maximum power draw of 65W and a nominal voltage requirement of 1.8V. 65W at 1.8V would equate to 36.1A of current at that voltage (volts amps = watts). If the voltage regulator used +5V as a feed, 65W would equate to only 13A at +5V. That would assume 100% efficiency in the regulator, which is impossible. Therefore, assuming 75% efficiency (it might be better, but I like to think conservatively), there would be about 17A actual draw on the +5V due to the regulator and processor combined.
20 1122 Chapter 21 Power Supply and Chassis/Case When you consider that the motherboard itself also uses some +5V power plus ISA or PCI cards are drawing power as well you can see how easy it is to overload the +5V lines from the supply to the motherboard. Although most motherboard VRM designs up through the Pentium III and Athlon/Duron use 5Vbased regulators, a transition is underway to use 12V-powered regulators. This is because the higher voltage will significantly reduce the current draw. As an example, using the same 65W AMD Athlon 1GHz CPU, you end up with the levels of draw at the various voltages shown in Table Table 21.6 Levels of Draw at Various Voltages Watts Volts Amps Amps at 75% Regulator Efficiency As you can see, using 12V to power the chip results in only 5.4A of draw, or 7.2A assuming 75% efficiency on the part of the regulator. So, modifying the motherboard VRM circuit to use the +12V power feed would seem simple. Unfortunately, the standard ATX 2.03 power supply design has only a single +12V lead in the main power connector. The auxiliary connector has no +12V leads at all, so that is no help. Pulling up to 8A more through a single 18ga. wire supplying +12V power to the motherboard is a recipe for a melted connector. To augment the supply of +12V power to the motherboard, Intel created a new ATX12V power supply specification. This adds a third power connector, called the ATX12V connector, specifically to supply additional +12V power to the board. This connector is shown in Figure (Black) GND (Black) GND +12V (Yellow) +12V (Yellow) Pin 4 ATX12V Power Connector Figure An ATX12V power connector. The pinout of the +12V power connector is shown in Table Table 21.7 ATX +12V Power Connector Pinout (Wire Side View) Color Signal Pin Pin Signal Color Yellow +12V 3 1 Gnd Black Yellow +12V 4 2 Gnd Black If you are replacing your motherboard with a new one that requires the ATX12V connection for the CPU voltage regulator, and yet your existing power supply doesn t have that connector, an easy solution is available. Merely convert one of the peripheral power connectors to an ATX12V type.
21 Motherboard Power Connectors Chapter PC Power and Cooling has released just such an adapter that can instantly make any standard ATX power supply into one with an ATX12V connector. The issue is not whether the power supply can generate the necessary 12V that has always been available via the peripheral connectors. The ATX12V adapter shown in Figure solves the connector problem quite nicely. Figure ATX12V adapter from PC Power and Cooling. ATX Optional Connector The ATX specification also defines an optional six-pin connector. This connector has two rows of three pins each to provide the signals and voltages. The computer can use these signals to monitor and control the cooling fan, monitor the +3.3V power to the motherboard, and provide power and grounding to IEEE 1394 (FireWire) devices. This connector has gone through several revisions in pinout since first being published, and I have yet to see any motherboards or power supplies on the market that actually support it. In fact, the latest ATX/ATX12V Power Supply Design Guide published by Intel states Details of the 2x3 Optional Power Connector mentioned in the ATX 2.03 Specification are omitted from this design guide until such time as the signals on that connector are more rigidly defined. Table 21.8 lists the pinout of the optional connector as defined in the ATX 2.03 Specification. Table 21.8 ATX Optional Power Supply Connections Color Signal Pin Pin Signal Color White/Black Stripe 1394R 4 1 FanM White White/Red Stripe 1394V 5 2 FanC White/Blue Stripe Reserved V sense White/Brown Stripe The FanM signal enables the operating system to monitor the status of the power supply s cooling fan so that it can take appropriate actions, such as shutting down the system if the fan fails. The motherboard (under operating system control) can use the FanC signal with variable voltages to control the operation of the power supply s fan, shifting it into a low power state or shutting it off completely when the system is in sleep or standby mode. The ATX standard specifies that a voltage of +1V or less indicates that the fan is to shut down, whereas +10.5V or more instructs the fan to operate at full speed. The system designer can define intermediate voltages to operate variable-speed fans at various levels. If the power supply does not include a variable-speed fan circuit, any voltage level higher than +1V on the FanC signal is interpreted as a command to run the fan at its full (and only) speed.
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